No you cannot directly update your repostitory from the external repostiory. You need to do vendor imports from your SorceForge checkouts. This is just as suggested in the version control article
but in step 2 of the "Setting up a new project under CVS" section instead of getting a new tarball, do a new checkout from SourceForge. Then continue with the vendor import into your projects's
The package system in OpenACS 4.0 should help you not have to import more code than you want. For update releases of the current OpenACS I would do vendor imports of the full source code - but realize that you will only get the diff stored in your CVS repository so you are not storing full versions of multiple tarballs.